AbstractLymph‐borne immunoblasts were labeledin vitrowith125I[ ] iodo‐deoxy‐uridine, washed and returned by intravenous injection to the sheep from which they had been collected. Twenty h later the sheep were killed and the distribution of the immunoblasts was determined by assaying the radioactivity in various organs.Immunoblasts from the efferent lymph of peripheral somatic lymph nodes (PSLN) went mainly t o the spleen, lungs and other PSLN, while immunoblasts from intestinal lymph went mainly to the small gut. This ability of intestinal immunoblasts t o home t o the gut was demonstrated also in the sterile environment of fetusesin utero; apparently the migratory behavior of immunoblasts, like that of small lymphocytes, is not primarily “antigen‐driven”.A technique was devised for the collection of peripheral (i.e.afferent to the mesenteric node) intestinal lymph which was found to contain 10–20 times the numbers of small lymphocytes that occur in the peripheral lymph from other tissues. Immunoblasts from peripheral intestinal lymph also homed to the gut.The immunoglobulin content of immunoblasts was studied by making detergent extracts of lymph cells, by applying immuno‐peroxidase techniques t o cell films and by investigating the incorporation of14C‐labeled amino acids into immunoglobulins by immunoblastsin vitro.Immunoblasts from both somatic and intestinal lymph contained and made IgG and IgM, but many intestinal immunoblasts contained and made IgA. It is not known whether this immunoglobulin mediates the extravasation of immunoblasts into the gut. Nonetheless, there is compelling evidence that there are two major migratory pathways for lymphoid cells; one through the gut‐associated lymphoid tissue and the other through the somatic‐spl